1. Field of the Invention
The present invention relates to an actuator, and in particular relates to an actuator for driving a lens of a digital camera and a mobile telephone with a camera feature.
2. Description of the Related Art
An actuator employing a piezoelectric element has been known as a driving device for a lens of a digital camera. For example, an actuator according to JP-A-2002-142470 is composed of a piezoelectric element, a driving member, and a pedestal base. A driving member is affixed to the end face at one side of the extension/contraction direction of the piezoelectric element, and a driven member is frictionally engaged with the driving member. Also, the pedestal base is affixed to the end face at the other side of the extension/contraction direction. The pedestal base has a function of a weight as an inertia body. Pulse-shaped voltage is applied to the piezoelectric element in such a construction, and movement of the piezoelectric element in the extension direction and the contraction direction is transmitted to the driving member by an inertia action of the pedestal base. Where the piezoelectric element is gradually deformed, the driven member moves along with the driving member, and where the piezoelectric element is rapidly deformed, the driven member remains at the same position by the inertia of the mass thereof. Therefore, by repeating application of pulse-shaped voltage, it is possible to cause the driven member to intermittently move at a minute pitch.
The thus composed actuator generates a problem of resonating among the pedestal base, the piezoelectric element, and the driving member. Further, the actuator according to JP-A-2002-142470 is resiliently supported on the main body by adhering the pedestal base to the main body with a rubber-based adhesive agent so that no influence due to resonance is given thereto when being mounted onto the main body. However, with such a drive system, it becomes very difficult to control the unevenness among the pedestal base, the piezoelectric element and the driving member.
If such resonance is utilized, there is an advantage by which the amount of movement of the driving member is increased. For example, Japanese Patent No. 3171187 describes an actuator utilizing the resonance. According to the actuator, by applying pulse-shaped voltage to displacement of the piezoelectric element when resonating, the amount of displacement of the driven member is increased.
However, since the actuator according to JP-A-2002-142470 or Japanese Patent No. 3171187 has a piezoelectric element, a driving member and a pedestal base (or a supporting member) disposed in line in the driving direction, the size thereof in the driving direction is increased, and where a particularly large pedestal base is used, there is a problem that the actuator becomes large. An actuator that can be downsized is demanded.
Further, since the actuator according to JP-A-2002-142470 or Japanese Patent No. 3171187 utilizes a resonant state generated inside the actuator composed of a piezoelectric element, a driving member and a pedestal base, there arises a problem that the driving member is adversely influenced by resonance, and the driving member is thus displaced in directions other than the extension/contraction direction of the piezoelectric element. For example, as depicted in FIG. 7A and FIG. 7B, there arises a problem that the driving member 2 is displaced in directions other than the extension/contraction direction upon receiving influence of resonance. Therefore, a driving force based on extension and contraction of the piezoelectric element 1 is not accurately transmitted to the driven member 3, wherein it becomes difficult to accurately cause the driven member 3 to move in the extension/contraction direction of the piezoelectric element 1. In particular, if the pedestal base (weight member) is made large to increase the thrust of the driven member 3, resonance is liable to occur, wherein an actuator in which resonance is hardly brought about is demanded.